The cause of Huntington?s disease is an increase in the trinucleotide CAG repeat from under 36 repeats to 36 or greater repeats. The mode for the number of repeats is 42, and most patients have between 40 and 45. The disease generally starts between ages 30 and 40, with onset and progression of impaired cognition, depression, and aberrant movement. The genetics is autosomal dominant. With gene editing, we aim to eliminate expression of the mutant allele of Huntington?s disease or repair the mutation by reducing the number of CAG repeats. We will use CRISPR-Cas9 for selective targeting of single nucleotide polymorphisms with heterozygosities to prevent the mutant huntingtin allele from producing protein. Generating frameshift mutations at the SNP heterozygosity reduces mutant huntingtin protein, with no production of mutant protein fragments. In a second approach, we reduce the number of CAG repeats from a high number in the mutant allele to a healthy number of CAG repeats, through the use of Cas9 nickases. The nickase Cas9D10A is used to reduce the CAG repeat number below the pathogenic threshold. We have preliminary data that supports each aspect of the proposal. The goal of this discovery application is to set the stage for promising therapeutics for treatment of Huntington?s disease and other autosomal dominant neurological disorders caused by trinucleotide CAG repeat expansions.